The visitors led 2-1 with four minutes left after a brilliant goal from Leigh Griffiths but United then levelled when Johnny Russell converted a spot-kick following a foul by Ryan McGivern on Gary Mackay-Steven.
Television replays showed the offence took place outside the penalty area but Fenlon said: "I've just watched it again and it's outside the box, but these things happen.
"We all make mistakes and we've just got to get on with it. The game is so quick.
"It's a good foot outside the box but we've just got to move on. It's important we take the positives out of today and not look at a mistake from the ref or whatever else.
"We can't do any more as a team than we did today. We created chances, we played really well, and we did enough to win the game.
"We defended well and it's outside of our control when things like that happen but what can we do?
"I haven't spoken to the referee, I'm not one that goes banging on refs' doors.
"It's a hard enough job without me doing that. You hope they balance themselves out over time.
"I'm disappointed that we didn't pick up three points but I'm chuffed with the performance.
"I thought we were very good today and I can't take anything away from the performance of the players."
Griffiths was excellent throughout but had to go off with 20 minutes remaining.
However, Fenlon said: "He should be okay. He's got a bit of a dead leg but I don't think he's too bad."